Collimating optical device and system

ABSTRACT

There is provided a light-guide, compact collimating optical device, including a light-guide having a light-waves entrance surface, a light-waves exit surface and a plurality of external surfaces, a light-waves reflecting surface carried by the light-guide at one of the external surfaces, two retardation plates carried by light-guides on a portion of the external surfaces, a light-waves polarizing beamsplitter disposed at an angle to one of the light-waves entrance or exit surfaces, and a light-waves collimating component covering a portion of one of the retardation plates. A system including the optical device and a substrate, is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.14/149,036, filed Jan. 7, 2014 for a Collimating Optical Device andSystem, now U.S. Pat. No. 9,448,408, granted Sep. 20, 2016 which is adivisional application of U.S. Ser. No. 12/596,823, filed on Feb. 17,2010 for a Collimating Optical Device and System, now U.S. Pat. No.8,643,948, granted Feb. 4, 2014.

FIELD OF THE INVENTION

The present invention relates to light-guide, compact collimatingoptical devices (LCCDs) and to optical systems which include one or moreof these devices. The term “light-guides” refers to anylight-transmitting body, preferably light-transmitting, solid bodies,also known as optical substrates.

The invention can be implemented to advantage in a large number ofimaging applications, such as head-mounted displays (HMDs) and head-updisplays (HUDs), cellular phones, compact displays, 3-D displays,compact beam expanders, as well as non-imaging applications, such asflat-panel indicators, compact illuminators and scanners.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical devices is in thefield of HMDs, wherein an optical module serves both as a reflectingoptical element and a combiner, in which a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), a scanning source orsimilar devices, or indirectly, by means of a relay lens or an opticalfiber bundle. The display comprises an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theviewer by means of a reflecting or partially reflecting surface actingas a combiner for non-see-through and see-through applications,respectively. Typically, a conventional, free-space optical module isused for these purposes. As the desired field-of-view (FOV) of thesystem increases, such a conventional optical module becomes larger,heavier, bulkier, and therefore, even for a moderate performance device,is impractical. This is a major drawback for all kinds of displays, butespecially in head-mounted applications, where the system mustnecessarily be as light and as compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications, and, on the other hand, suffermajor drawbacks in terms of manufacturability. Furthermore, theeye-motion-box (EMB) of the optical viewing angles resulting from thesedesigns is usually very small—typically, less than 6 mm. Hence, theperformance of the optical system is sensitive, even for small movementsof the optical system relative to the eye of the viewer, and does notallow sufficient pupil motion for comfortable reading of text from suchdisplays.

The teachings included in the Publications WO 01/95027, WO 2006/013565,WO 2006/085307, WO 20061/085310, WO 2007/054928, and WO 2008/023367 inthe name of Applicant, are herein incorporated by references.

DISCLOSURE OF THE INVENTION

The present invention facilitates the design and fabrication of acompact imaging device for, among other applications, HMDs. Theinvention allows for relatively wide FOVs together with relatively largeEMB values. The resulting optical system offers a large, high-qualityimage, which also accommodates large movements of the eye. The opticalsystem offered by the present invention is particularly advantageousbecause it is substantially more compact than state-of-the-artimplementations and yet it can be readily incorporated, even intooptical systems having specialized configurations.

The invention also enables the construction of improved HUDs. Since theinception of such displays more than three decades ago, there has beensignificant progress in the field. HUDs have indeed become popular andthey now play an important role, not only in most modern combataircraft, but also in civilian aircraft, in which HUD systems havebecome a key component for low-visibility landing operation.Furthermore, there have recently been numerous proposals and designs forHUDs in automotive applications, where they can potentially assist thedriver in driving and navigation tasks. State-of-the-art HUDs,nevertheless, suffer several significant drawbacks. All HUDs of thecurrent designs require a display light source that must be offset asignificant distance from the combiner to ensure that the sourceilluminates the entire combiner's surface. As a result, thecombiner-projector HUD system is necessarily bulky, large and requiresconsiderable installation space, which makes it inconvenient forinstallation and at times even unsafe to use. The large optical apertureof conventional HUDs also pose a significant challenge for the opticaldesign, either rendering the HUDs with compromising the performance, orleading to high cost wherever high-performance is required. Thechromatic dispersion of high-quality holographic HUDs is of particularconcern.

A broad object of the present invention is therefore to alleviate thedrawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

A further object of the present invention relates to its implementationin a compact HUD, alleviating the aforementioned drawbacks. In the HUDdesign of the current invention, the combiner is illuminated with acompact display light source that can be attached to the substrate.Hence, the overall system is very compact and can readily be installedin a variety of configurations for a wide range of applications. Inaddition, the chromatic dispersion of the display is negligible and, assuch, can operate with wide spectral sources, including a conventionalwhite-light source. In addition, the present invention expands the imageso that the active area of the combiner can be much larger than the areathat is actually illuminated by the light source.

A still further object of the present invention is to provide a compactdisplay with a wide FOV for a mobile, hand-held application such ascellular phones and personal display modules. In today's wirelessinternet-access market; a sufficient bandwidth is available for fullvideo transmission. The limiting factor remains the quality of thedisplay within the device of the end-user. The mobility requirementrestricts the physical size of the displays, and the result is adirect-display with poor image viewing quality. The present inventionenables a physically very compact display with a very large virtualimage. This is a key feature in mobile communications, and especiallyfor mobile internet access, solving one of the main limitations for itspractical implementation. Thereby, the present invention enables theviewing of a digital content of a full format internet page within asmall, hand-held device, such as a cellular phone.

In accordance with the invention, there is therefore provided, alight-guide, compact collimating optical device, comprising alight-guide having at least one light-waves entrance surface, at leastone light-waves exit surface and a plurality of external surfaces, atleast one light-waves reflecting surface carried by the light-guide atone of said external surfaces, two or more retardation plates carried bylight-guides on at least a portion of said external surfaces, at leastone light-waves polarizing beamsplitter disposed at an angle to at leastone of said light-waves entrance or exit surfaces, and at least onelight-waves collimating component covering at least a portion of atleast one of said retardation plates.

The invention further provides an optical system comprising alight-guide, compact collimating optical device, comprising alight-guide having at least one light-waves entrance surface, at leastone light-waves exit surface and a plurality of external surfaces, atleast one light-waves reflecting surface carried by the light-guide atone of said external surfaces, two or more retardation plates carried bylight-guides on at least a portion of said external surfaces, at leastone light-waves polarizing beamsplitter disposed at an angle to at leastone of said light-waves entrance or exit surfaces, and at least onelight-waves collimating component covering at least a portion of atleast one of said retardation plates and further comprising alight-waves transmitting substrate having at least two major surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 is a schematic diagram illustrating an optical device forcollimating input light-waves from a display light source, in accordancewith the present invention;

FIG. 2 is a schematic diagram illustrating a system for collimating andcoupling-in input light-waves from a display light source into asubstrate, in accordance with the present invention;

FIG. 3 is a schematic diagram illustrating another system forcollimating and coupling-in input light-waves from a display lightsource into a substrate, wherein the collimating module is cemented tothe substrate, in accordance with the present invention;

FIGS. 4 and 5 are graphs illustrating reflectance curves as a functionof incident angles, for an exemplary angular sensitive coating for s-and p-polarized light-waves, respectively;

FIG. 6 is a schematic diagram illustrating yet another embodiment forcollimating and coupling-in input light-waves from a display lightsource into a substrate utilizing a half-wavelength retardation plate,in accordance with the present invention;

FIG. 7 is a schematic diagram illustrating still a further embodimentfor collimating and coupling-in input light-waves from a display lightsource into a substrate utilizing angular sensitive coating, inaccordance with the present invention;

FIG. 8 is a schematic diagram illustrating an optical device forcollimating input light-waves from a display light source utilizing ablank plate, in accordance with the present invention;

FIG. 9 is a schematic diagram illustrating a device for collimatinginput light-waves from a display light source by utilizing two lensesand blank plate, in accordance with the present invention;

FIG. 10 is a schematic diagram illustrating a device for collimatinginput light-waves from a display light source by utilizing two lenses,in accordance with the present invention;

FIG. 11 is a schematic diagram illustrating a device for collimatinginput light-waves from liquid crystals on silicon (LCOS) light source,in accordance with the present invention;

FIG. 12 is a schematic diagram illustrating another device forcollimating input light-waves from an LCOS light source, in accordancewith the present invention;

FIG. 13 is a schematic three-dimensional diagram illustrating yetanother device for collimating input light-waves from an LCOS lightsource, in accordance with the present invention;

FIG. 14 is a schematic diagram illustrating a system for collimatinginput light-waves and coupling-in input light-waves from an LCOS lightsource into a substrate, in accordance with the present invention;

FIG. 15 is a schematic diagram illustrating another system forcollimating input light-waves and coupling-in input light-waves from anLCOS light source into a substrate, in accordance with the presentinvention;

FIG. 16 is a schematic diagram illustrating an optical system forcollimating input light-waves from a display light source utilizing afolding prism, in accordance with the present invention;

FIG. 17 is a schematic diagram illustrating an optical device forcollimating input light-waves from an LCOS light source utilizing afolding prism, in accordance with the present invention;

FIG. 18 illustrates a span of optical rays coupled into a substrate;

FIG. 19 is a schematic diagram illustrating a system for coupling lightinto a substrate, in accordance with the present invention;

FIG. 20 is a schematic diagram illustrating a system for coupling lightinto a substrate utilizing a coupling prism, in accordance with thepresent invention;

FIG. 21 illustrates an optical system for collimating and coupling-ininput light-waves from a single display light source into two separatesubstrates, in accordance with the present invention;

FIG. 22 illustrates a front view of an embodiment of an optical systemfor collimating and coupling-in input light-waves from a single displaylight source into two separate substrates, in accordance with thepresent invention;

FIG. 23 illustrates an optical system for collimating and coupling-ininput light-waves from two display light sources into two separatesubstrates, in accordance with the present invention;

FIG. 24 illustrates another optical system for collimating andcoupling-in input light-waves from two display light sources into twoseparate substrates, utilizing an optical beamsplitter, in accordancewith the present invention;

FIG. 25 illustrates an optical system for collimating and coupling-ininput light-waves from a single display light source into two separatesubstrates, utilizing a polarizing beamsplitter and a dynamichalf-wavelength retardation plate, in accordance with the presentinvention;

FIG. 26 is a schematic diagram illustrating an optical device forcollimating input light-waves from a display light source, wherein aghost image appears in the output image;

FIG. 27 is a schematic diagram illustrating an optical device forcollimating input light-waves from a display light source, utilizing twopolarizing beamsplitters and a linear polarizer, in accordance with thepresent invention;

FIG. 28 is a schematic diagram illustrating an optical device forcollimating input light-waves from a display light source, wherein theoutput light-waves are rotated compared to the input light-waves, inaccordance with the present invention;

FIG. 29 is a schematic diagram illustrating another optical device forcollimating input light-waves from a display light source, wherein theoutput light-waves are rotated compared to the input light-waves, inaccordance with the present invention;

FIG. 30 is a schematic diagram illustrating an optical device forcollimating input light-waves from an LCOS light source, wherein theoutput light-waves are rotated compared to the input light-waves, inaccordance with the present invention;

FIG. 31 is a schematic diagram illustrating another optical device forcollimating input light-waves from an LCOS light source, wherein theoutput light-waves are rotated compared with the input light-waves, inaccordance with the present invention;

FIG. 32 is a schematic diagram of an optical device for collimatinginput light-waves from an LCOS light source, including a beamsplitterfor exclusively illuminating the display source, in accordance with thepresent invention;

FIG. 33 is a schematic diagram illustrating yet another embodiment of anoptical device for collimating input light-waves from an LCOS lightsource, including a beamsplitter for exclusively illuminating thedisplay source, in accordance with the present invention;

FIG. 34 is a schematic diagram illustrating an optical device forcollimating and coupling-in input light-waves from a display lightsource into a substrate, wherein the output light-waves are rotatedcompared to the input light-waves, in accordance with the presentinvention;

FIG. 35 is a schematic diagram illustrating another embodiment of anoptical device for collimating and coupling-in input light-waves from adisplay light source into a substrate, wherein the output light-wavesare rotated compared with the input light-waves, in accordance with thepresent invention;

FIG. 36 is a schematic diagram illustrating an optical device forcollimating and coupling-in input light-waves from an LCOS light sourceinto a substrate, wherein the output light-waves are rotated comparedwith the input light-waves, in accordance with the present invention;

FIGS. 37 to 39 are, respectively, top, side and three-isometric views ofan optical device for collimating and coupling-in input light-waves froma display light source into a substrate, in accordance with the presentinvention;

FIG. 40 is a schematic diagram illustrating an optical device forcollimating input light-waves from a display light source into asubstrate, wherein the collimating optical device and the substrate arefabricated from different optical materials, in accordance with thepresent invention;

FIG. 41 is a schematic diagram illustrating the distortion effect whenthe collimating optical device and the substrate are fabricated fromdifferent optical materials;

FIG. 42 is a schematic diagram illustrating a system for compensatingfor the chromatic aberration of an optical system which is fabricatedfrom two different materials, in accordance with the present invention;

FIG. 43 is a schematic diagram illustrating a system for compensatingfor the distortion of an optical system which is fabricated from twodifferent materials, in accordance with the present invention;

FIG. 44 is a three-dimensional schematic diagram illustrating a systemfor compensating for the chromatic aberration of an optical system whichis fabricated from two different materials, in accordance with thepresent invention;

FIG. 45 is a schematic diagram illustrating the chromatic aberrationwhen a central trapped light-wave is coupled out from the substrate atan output direction which deviates with respect to the normal of themajor plane of the substrate;

FIG. 46 is a schematic diagram illustrating distortion when a centraltrapped light-wave is coupled out from the substrate at an outputdirection which deviates with respect to the normal of the major planeof the substrate;

FIG. 47 is a schematic diagram illustrating a system for compensatingfor the chromatic aberration of an optical system when the outputdirection deviates with respect to the normal of the major plane of thesubstrate, in accordance with the present invention;

FIG. 48 is a schematic diagram illustrating an optical device forcompensating for the distortion of an optical system when the outputdirection deviates with respect to the normal of the major plane of thesubstrate, in accordance with the present invention;

FIG. 49 is a schematic diagram illustrating a device for collimatinginput light-waves from a display light source into an substrate, byutilizing a telephoto system, in accordance with the present invention;

FIG. 50 is a schematic diagram illustrating an optical device having afocusing lens, in accordance with the present invention;

FIG. 51 is a schematic diagram illustrating an optical system having aunity magnification telescope, in accordance with the present invention,and

FIG. 52 is a schematic diagram illustrating an optical system having aunity magnification telescope, including an inverting prism inaccordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following there will be described superior structures of opticaldevices that are more compact than the prior art devices, having variousconfigurations suitable for matching preferred outlines of opticalsystems, while still maintaining desired optical properties of thesystem. In such a structure, which exploits the fact that in mostmicrodisplay light sources such as LCDs or LCOS light sources, the lightis linearly polarized, as illustrated in FIG. 1. As shown, thes-polarized input light-waves 2 from the display light source 4 arecoupled into a light-guide (optical device) 6, which is usually composedof a light-waves transmitting material, through its entrance surface 8.Following reflection-off of a polarizing beamsplitter 10, thelight-waves are coupled-out of the substrate through an external surface12 of the light-guide 6. The light-waves then pass through aquarter-wavelength retardation plate 14, reflected by a reflectingoptical element 16, e.g., a flat mirror, return to pass again throughthe retardation plate 14, and re-enter the light-guide 6 throughexternal surface 12. The now p-polarized light-waves pass through thepolarizing beamsplitter 10 and are coupled out of the light-guidethrough an external surface 18 of the light-guide 6. The light-wavesthen pass through a second quarter-wavelength retardation plate 20,collimated by a component 22, e.g., a lens, at its reflecting surface24, return to pass again through the retardation plate 20, and re-enterthe light-guide 6 through external surface 18. The now s-polarizedlight-waves reflect-off the polarizing beamsplitter 10 and exit thelight-guide through the exit surface 26. The reflecting surfaces 16 and24 can be materialized either by a metallic or by a dielectric coating.

FIG. 2 illustrates how a LCCD 28 constituted by the components detailedwith respect to FIG. 1, can be combined with a substrate 30, to form anoptical system. Such a substrate 30 typically includes at least twomajor surfaces 32 and 34 and edges, one or more partially reflectingsurface 36 and an optical element 38 for coupling light thereinto. Theoutput light-waves 40 from the LCCD 28 enter the substrate 30 throughits lower surface 32. The incoming light-waves (vis-a-vis the substrate30) are reflected from optical element 38 and trapped in the substrateas illustrated in FIG. 2. Now, the LCCD 28, comprising the display lightsource 4, the folding prisms 42 and 44, the polarizing beamsplitter 10,the retardation plates 14 and 20 and the reflecting optical element 16and reflecting collimating component 22, can easily be integrated into asingle mechanical module which can be assembled independently of thesubstrate, with fairly relaxed mechanical tolerances. In addition, theretardation plates 14 and 20 and the reflecting optical elements 16 and22 could be cemented together respectively to form single elements.Alternatively, other methods could be used to combine these into asingle element, such as laminating a quarter-wavelength film onto thefront surface of the reflecting optical element 16 and reflectingcollimating component 22. Furthermore, all the optical elements of theLCCD 28, apart from the display light source 4, could be cementedtogether to form a single optical module. Regarding the display lightsource, it is usually required to keep an air gap between the LCCD andthe display light source, in order to enable a focusing mechanism,however, there are systems wherein the required focal distance of theLCCD is known. For instance, for optical systems wherein the imagelight-waves are coupled into a substrate, the optical waves should becollimated by the LCCD to infinity. If, in addition, the focal depth ofthe collimating component is large enough to accommodate the variousfabrication and assembly tolerances of the LCCD, then it is possible tolocate the display light source 4 with regard to the LCCD 28 at apre-defined distance. In that case, the display source could beattached, e.g., cemented, to the LCCD 28 utilizing an intermediatetransparent plate 46 (FIG. 3), having a thickness according to therequired focal distance of the LCCD.

It would be advantageous to attach all the various components of theLCCD 28 to the substrate 30, to form a single compact element with amuch simpler mechanical module. FIG. 3 illustrates such a module whereinthe exit surface 26 of the LCCD 28 is cemented, at the interface plane48, to the lower surface 32 of the substrate 30. The main problem ofthis configuration is that the attaching procedure cancels thepreviously existing air gap 50 (illustrated in FIG. 2) between thesubstrate 30 and the LCCD 28. This air gap is essential for trapping theinput light-waves 40 inside the substrate 30. As illustrated in FIG. 3,the trapped light-waves 40 should be reflected at points 52 and 54 ofthe interface plane 48. Therefore, a reflecting coating should beapplied at this plane, either at the major surface 32 of the substrate30 or at the upper exit surface 26 of the LCCD 28. A simple reflectingcoating cannot, however, be easily applied, since these surfaces shouldalso be transparent to the light-waves that enter and exit the substrate30 at the exemplary points 56. The light-waves should pass through plane48 at small incident angles, and reflect at higher incident angles. Inthe example illustrated, the passing incident angles are between 0° and15° and the reflecting incident angles are between 47° and 80°.

FIGS. 4 and 5 illustrate, for s- and p-polarization respectively, thereflectance curves as functions of the incident angles for threerepresentative wavelengths in the photopic region: 460 nm, 550 nm and640 nm. As shown in FIG. 4, it is possible to achieve the requiredbehavior of high reflectance (above 95%) at large incident angles andlow reflectance (below 5%) at small incident angles, for s-polarizedlight-waves. For p-polarized light-waves, however, as illustrated inFIG. 5, it is impossible to achieve high reflectance at incident anglesbetween 50° and 70°, due to the proximity to the Brewster angle.

In the system illustrated in FIG. 3, the light-waves from the displaylight source, as well as the reflected light-waves of the couplingmirror 38, which impinge on the points 52 and 54, are s-polarized andthe required reflectance could be achieved. There are situations,however, where the light-waves from the display light source arelinearly p-polarized and the major axis of the grid is rotated by 90°,compared to that of FIGS. 1 and 2, i.e., the polarizing beamsplitter isoriented here to reflect the p-polarization and transmit thes-polarization.

FIG. 6 illustrates a device wherein a half-wavelength retardation plate58 is disposed between the upper surface 26 of the LCCD 28 and the exitsurface 32 of the substrate 30. When passing through the plate 58 thepolarization of the light-waves is rotated and the now s-polarizedlight-waves are coupled into the substrate.

A difficulty still existing in the configurations of FIGS. 3 and 6 isthat the substrate 30 and the LCCD 28, are assembled from severaldifferent components. Since the fabrication process usually involvescementing optical elements, and, since the required angular-sensitivereflecting coating 27 is applied to the surface of the light-guide onlyafter the bodies of the substrate 30 and the LCCD 28 are complete, it isnot possible to utilize the conventional hot-coating procedures that maydamage the cemented areas. Novel thin-film technologies, as well asion-assisted coating procedures, can also be used for cold processing.Eliminating the need to heat parts, allows cemented parts to be coatedsafely.

An alternative structure of a device is illustrated in FIG. 7. Here,transparent plate 60 is placed at the interface plane 48. Now, therequired coating can simply be applied to the upper surface of thislight-guide, which is adjacent to the substrate 30, utilizingconventional hot-coating procedures and then cementing it at the properplace.

In the devices illustrated in FIGS. 1 to 3 and 6 to 7, it is assumedthat the optical path inside the LCCD 28 from the display light source 4to the reflecting surface 24 of the component 22 is the required focallength of the collimating component. Usually, this can be achieved bycontrolling the lateral dimensions of the LCCD 28, however, there aresystems, particularly for eyeglasses systems, where it is desired tominimize the lateral dimension between surface 12 and surface 18 of theLCCD 28. That is, to reduce the width of the LCCD 28 at the expense ofincreasing the distance between the display light source and thesubstrate 30. The lateral dimensions of the folding prism could bereduced, as long as the entire FOV of the image can be coupled into theLCCD with no vignetting. In that case, however, the optical path betweenthe display light source 4 and the collimating component 22 is reduced,and therefore, the output light-waves from the LCCD 28 are no longercollimated.

As illustrated in FIG. 8, in order to compensate for this defocusing, ablank plate 62 is placed between the display light source 4 and thelower prism 42 of the LCCD 28. Preferably, in order to simplify thefinal assembly of the system, as explained above with reference to FIGS.6 and 7, the plate 62 is optically cemented to the lower prism 42 at theinterface plane 64. As shown, even though the lateral dimension of theLCCD 28 has been reduced, the marginal waves 66 and 68 are collimated bythe LCCD 28 to the exit pupil 70, without any obstruction.

In the systems illustrated hereinbefore, only a single sphericalconverging lens is utilized. For some optical schemes that may besufficient, however, for other systems with wide FOVs and large inputapertures, a better optical quality may be required. One approach toimprove the optical properties of the system is to exploit eitheraspheric or even aspheric-diffractive lenses. Another approach is toutilize more than one reflecting optical element.

In the optical device shown in FIG. 9, the planar reflecting surface 16(of FIG. 1) is replaced by a second converging lens 72. The light-wavesthat pass through the quarter-wavelength retardation plate 14 of FIG. 1are partially collimated by collimating component 72, e.g., a lens, atits reflecting surface 74. The partially collimated light-waves returnto pass again through the retardation plate 14, and reenter thelight-guide 6 through surface 12. The now p-polarized light-waves passthrough the polarizing beamsplitter 10 and are coupled-out of thelight-guide through surface 18 of the light-guide. The light-waves thenpass through the second quarter-wavelength retardation plate 20, and arecompletely collimated by the component 22 at its reflecting surface 24.Another benefit of utilizing two collimating components in the LCCD 28is that the required focal length can be achieved with a shorter opticalpath between the display source and the output surface of the LCCD 28,by exploiting an appropriate telephoto design.

FIG. 10 illustrates an optical device with two converging collimatingcomponents 22 and 72, wherein it is not necessary to utilize the blankplate 62 (of FIG. 8). This device is shorter than the one shown in FIG.8 as well as narrower than the one which shown in FIG. 1.

Another advantage of the proposed imaging device illustrated heremanifests itself when utilizing an LCOS light source device as thedisplay light source. Like LCD panels, LCOS light source panels containa two-dimensional array of cells filled with liquid crystals that twistand align in response to control voltages. With the LCOS light source,however, the cells are grafted directly onto a reflective silicon chip.As the liquid crystals twist, the polarization of the light is eitherchanged or unchanged following reflection of the mirrored surface below.This, together with a polarizing beamsplitter, causes modulation of thelight-waves and creates the image. In addition, the reflectivetechnology means the illumination and imaging light beams share the samespace. Both of these factors necessitate the addition of a specialbeamsplitting optical element to the module, in order to enable thesimultaneous operations of the illuminating as well as the imagingfunctions. The addition of such an element would normally complicate themodule and, when using an LCOS light source as the display light source,some modules using a frontal coupling-in element or a folding prismwould become even larger. For the imaging devices illustrated in FIGS. 8to 10, however, it is readily possible to add the illuminating unit tothe module without significantly increasing the volume of the systemincorporating the device.

FIG. 11 illustrates a cubic polarizing beamsplitter 80 inserted insteadof a simple blank plate between the display light source and the LCCD28. Here, the p-polarized light-waves 82, emanating from a light source84, reflect-off the polarizing beamsplitter 86 and illuminate the frontsurface of the LCOS light source 88. The polarization of the reflectedlight-waves from the “light” pixels is rotated to the s-polarization andthe light-waves are then passed through the beamsplitter 86 and enterthe prism 42 through the lower surface 8. The light-waves are thencollimated as described above with reference to FIG. 1. If thelight-waves source 84 is unpolarized, it is possible to add a polarizer90, which transmits only the desired polarization. The LCCD 28 remainscompact and retains its narrow form.

A modified embodiment of FIG. 11, wherein a longer optical pathwayinside the LCCD is required, is illustrated in FIG. 12. Here, thep-polarized light-waves 82, emanating from a light source 84, passthrough the polarizing beamsplitter 86 and illuminate the front surfaceof the LCOS light source 88. The polarization of the reflectedlight-waves from the “light” pixels is rotated to the s-polarizationstate. Following reflection-off, the polarizing beamsplitter 86, thelight-waves are coupled-out of the light-guide 80 through the lowersurface 92 of the light-guide. The light-waves then pass through aquarter-wavelength retardation plate 94, reflected by a reflectingoptical element 96, return to pass again through the retardation plate94, and re-enter the light-guide 80 through the lower surface 92. Thenow p-polarized light-waves pass through the polarizing beamsplitter 86,and enter the upper prism 42 through the lower surface 8. Thelight-waves are then collimated, as described above with reference toFIG. 1. If it is required that the coupled light-waves 82 into the prism42 be s-polarized as previously, it is possible to add a half-wavelengthretardation plate 98 between prisms 80 and 42, which rotates thelight-waves into the desired polarization. The reflecting surfaces 16and 96 could be replaced by converging optical components, as explainedabove with reference to FIG. 9.

In the optical devices illustrated in FIGS. 11 and 12, the optical pathis folded around the y-axis, i.e., in the x-z plane. Usually, it ispreferable to fold the optical path in the plane where the dimensions ofthe device are minimal. Assuming the configuration of eyeglass systems,the x and y axes refer to the horizontal and vertical axes of the image.For most of the substrate-based eyeglasses configurations, the verticaldimension of the input aperture of the substrate, vis-a-vis the verticaloutput aperture of the LCCD, is considerably larger than the horizontaldimension of the aperture. Hence, it is preferred to fold the opticalpathway in prism 42, which is adjacent to the substrate, around they-axis, as is illustrated indeed in these Figures. Regarding the displaylight source, however, the situation is usually the opposite, i.e., thehorizontal dimension is larger than the vertical one in a ratio of 4:3for VGA format and in a ratio of 16:9 for HDTV format. It is thereforepreferred to fold the optical pathway in the prism 80, which is adjacentto the display source, around the x axis. A modified embodiment of FIG.11, wherein the folding of the optical pathway is performed in twodifferent planes, is illustrated in FIG. 13. Here the folding-in prism80 is performed in the y-z plane, while in prism 42 it is performed inthe x-z plane. In this configuration, the light 82 coupled into theprism 80 which is p-polarized compared to the beamsplitter 86 will bes-polarized compared to the folding beamsplitter 10, and no rotatinghalf-wavelength retardation plate is required between prisms 80 and 42.

For eyeglasses configurations it is usually required that the longerdimension of the LCCD be oriented along the handle of the eyeglasses,i.e., normal to the major surfaces of the substrate, as illustrated inthe preceding figures. For other configurations, however, such ashand-held displays, it is required that the longer dimension of the LCCDwill be oriented parallel to the major surfaces of the substrate.

As illustrated in FIG. 14, the s-polarized light-waves 82, emanatingfrom a light source 84, are reflected-off the first polarizingbeamsplitter 86 and illuminate the front surface of the LCOS lightsource 88. The polarization of the reflected light-waves from the“light” pixels is rotated to the p-polarization state. Following apassage through the polarizing beamsplitter 86, the light-waves arecoupled-out of the prism 100 through the lower surface 92 of the prism.The light-waves then pass through a quarter-wavelength retardation plate94, reflect by a reflecting element 96, return to pass again through theretardation plate 94, and re-enter the prism 100 through the lowersurface 92. Following the reflection-off the first polarizingbeamsplitter 86 and the second beamsplitter 102, the now s-polarizedlight-waves are coupled-out of the prism 100 through the lower surface92 of the prism. The light-waves then pass through a secondquarter-wavelength retardation plate 104, collimated by a component 106at its reflecting surface 108, return to pass again through theretardation plate 104, and re-enter the prism 100 through the lowersurface 92. The now-p-polarized light-waves pass through the polarizingbeamsplitter 102 and exit the LCCD through the upper surface 110 toenter the substrate 30. If it is required that the coupled light-waves82 into the substrate 30 be s-polarized as previously, it is possible toadd a half-wavelength retardation plate 58 between the upper surface 110of the LCCD 80 and the lower surface 32 of the substrate 30, whichrotates the light-waves into the desired polarization.

A modified embodiment of FIG. 14, wherein the two beamsplitters areoriented parallel to each other, is illustrated in FIG. 15. Here,instead of a right-angle prism 100, a parallelepiped 112 connects thetwo beamsplitters 86 and 102. As a result, the LCOS light source 88 isnot located on the same side of the LCCD as the substrate 30, but on theother side. This modification is required for systems where it is notallowed to locate the PCB of the LCOS light source near the substrate,due to assembly considerations.

In all the eyeglasses configurations illustrated above, the displaysource plane is oriented parallel to the major surfaces of thesubstrate. There are systems however, mainly with display sources havingPCB with large area, where it is required that the display source planebe oriented normal to the major surfaces of the substrate.

FIG. 16 illustrates a modified LCCD 114 containing two embeddedpolarizing beamsplitters 116 and 118, two quarter-wavelength retardationplates 120 and 122, two reflecting surfaces 124 and 126 and a converginglens 128. As illustrated, the s-polarized input light-wave 130 from thedisplay source 132 reflects off the first reflecting surface 124. Then,following total internal reflection off the left surface 134 of the LCCD114, the light-waves are reflected off the first beamsplitter 116 andare coupled out. It is then reflected and changed to p-polarized lightby the retardation plate 120 and the second reflecting surface 126.Following a passage through the polarizing beamsplitters 116 and 118,the light-waves are reflected, collimated and changed back tos-polarized light by the retardation plate 122 and the converging lens128. The light-waves are then reflected off the second polarizingbeamsplitter 118 and exit the LCCD through the upper surface 136. Theincoming light-waves are now trapped into the substrate 30 in the samemanner as illustrated in FIG. 2.

As illustrated in FIG. 17, in the event where the display light sourceis an LCOS light source device, the illumination will be changed byadding a complementary prism 138 with an embedded polarizingbeamsplitter 139 (instead of the reflecting surface 124 of FIG. 16), anda front illumination module 140.

For all the optical configurations which are illustrated herein, thecoupling-in of the light-waves into the substrate is performed utilizinga reflecting surface 38 embedded in the substrate. This coupling-inmethod, however, suffers from a few major drawbacks. Firstly, since thereflecting surface 38 and the partially reflecting surfaces 36 areoriented at different angles, the fabrication process of the substratehaving an internally embedded reflecting mirror 38, is rathercomplicated. In addition, the distance l (FIG. 2) between the input andthe output apertures of the substrate, which are set by the coupled-inmirror 38 and the coupled-out surfaces 36 respectively, is determined bythe fabrication process of the substrate. It is therefore not possibleto control the distance l for a given substrate and for manyapplications it is required to keep l as a flexible parameter. Forexample, in eyeglasses configuration, the distance l depends on the sizeand the shape of the viewer's head and on the particular model of theeyeglasses frame. It is therefore advantageous to have the ability toset l during the assembly process, since otherwise, it is required tomanufacture the substrates with a large variety of sizes in order toaccommodate all the required possibilities. As a result, a simplercoupling-in configuration is preferred as compared to that which isillustrated hereinabove.

The objective is therefore to find an alternative coupling-in mechanismwhich will replace the input mirror 38. FIG. 18 illustrates a span ofrays that are intended to be coupled into the substrate with a minimalrequired input aperture. In order to avoid an image with gaps orstripes, the points on the boundary line 146 between the edge of inputaperture 148 and the lower surface 150 of the substrate 30, should beilluminated for each one of the input light-waves by two different raysthat enter the substrate from two different locations: one ray 152 athat illuminates the boundary line 146 directly, and another ray 152 b,which is first reflected by the upper surface 154 before illuminatingthe boundary line. The size of the input aperture is usually determinedby two marginal rays: the rightmost ray 156 b of the highest angle ofthe FOV and the leftmost ray 158 a of the lowest angle of the FOV.

The simplest way to couple these rays into the substrate is illustratedin FIG. 19. Here, the principal axis 160 of the LCCD 28 is oriented atthe required off-axis angle α compared to the major plane of thesubstrate. A relay prism 162 is located between the LCCD 28 and thesubstrate 30 and is optically cemented to the lower surface 32 of thesubstrate such that the light from the display source is trapped insidethe substrate by total internal reflection. The overall shape and sizeof this module conforms to most of the relevant applications. Here, theoptical component for coupling light-waves into the substrate is nolonger a part of the substrate, but rather a part of the LCCD.Specifically, since the light-waves are reflected from the polarizingbeamsplitter 10 directly into the substrate at an oblique angle whichcouples the light-waves inside the substrate by total internalreflection, it can be related to as the coupling-in element.

In the optical collimating module illustrated in FIG. 19, the off-axisangle of the span of rays that are intended to be coupled into thesubstrate, are set by rotating the LCCD module. There are, however,cases where it is required to utilize collimated light-waves thatimpinges the substrate, normal to the substrate plane. In these cases analternative coupling-in mechanism should replace the input mirror 38.

As illustrated in FIG. 20, the lower surface 168 of a coupling-in prism170 is optically cemented to the substrate 30 at the upper surface 154of the substrate. The collimated light-waves from the collimatingoptical device (not shown) pass through the substrate 30 and the prism170 and are then reflected from the reflecting surface 172. Afterpassing again through the prism 170 the light-waves are coupled into thesubstrate by total internal reflection. Similarly to what is illustratedabove in FIG. 18, in order to avoid an image with gaps or stripes, thepoints on the boundary line 176 between the lower surface 168 of theprism 170 and the upper surface 154 of the substrate 30 should beilluminated for each one of the input light-waves by two different raysthat enter the substrate from two different locations: one ray 152 afirst passes through prism 170 and is reflected by the reflectingsurface 172, from there it illuminates the boundary line 176. Anotherray 152 b, is first reflected by the reflecting surface 172 and then bythe lower surface 150 of the substrate 30 before illuminating theboundary line. To avoid undesired reflections from the left surface 174it can be coated by an opaque obstructive layer.

There are a few alternatives as to the precise way in which a substratecan be utilized with the embodiments directed to the eyeglassesconfiguration. The simplest option is to use a single element for oneeye. Another option is to use an element and a display source for eacheye, projecting the same image, wherein the preferred place for the LCCDmodules is next to the temples. A similar option is to project the sameimage for both eyes but utilize only one LCCD which is located betweenthe two glasses, whereby its output is split between the two substrates.Alternatively, it is possible to project two different parts of the sameimage, with some overlap between the two eyes, enabling a wider FOV. Yetanother possibility is to project two different scenes, one directed toeach eye, in order to create a stereoscopic image. With thisalternative, attractive implementations are possible, including threedimensional movies, advanced virtual reality, training systems, and thelike.

An embodiment of a double-image arrangement, containing a single displaylight source 180, two LCCDs 182R and 182L, two coupling-in prisms 184Rand 184L and two substrates 30R and 30L is illustrated in FIG. 21. Thecollimating of the right light-waves by the LCCD 182R and the couplingin by the prism 184R into the substrate 30R is similar to thatillustrated above with reference to FIGS. 2 and 20, respectively. Themain difference in the LCCD 182R is that instead of utilizing a simplereflecting mirror, a half-reflecting beamsplitter 186 is localizedbetween the LCCDs 182R and 182L. As illustrated, the light-waves whichpartially pass through the beamsplitter 186 changed to p-polarized lightby passing through two quarter wave retardation plates 188R and 188L,which are positioned between the beamsplitter 186 and LCCDs 182R and182L, respectively. The p-polarized light passes through the polarizingbeamsplitter 190L. The light-waves are then reflected, collimated andchanged back to s-polarized light by the retardation plate 192L andconverging lens 194L. Following a reflection off the polarizingbeam-splitter 190L, the light-waves are reflected and changed backtop-polarized light by the retardation plate 196 and the reflectingsurface 198. The light-waves then pass through the polarizingbeamsplitter 190L and exit the substrate through the upper surface 200L.The incoming waves could be trapped into the substrate in the samemanner as that illustrated in FIG. 20.

As illustrated, the polarization of images 202L and 202R which areformed by the LCCDs 182L and 182R are p and s, respectively, which mightbe a shortcoming for systems where a similar polarization is requiredfrom both images. A half-wavelength retardation plate 204 is insertedbetween the left LCCD 182L and the left substrate 30L to form twoidentical linearly s-polarized images, 206L and 206R, which areprojected into the viewer's eyes 208L and 208R, respectively. In orderto form a proper binocular image, it is essential that the images 206Land 206R be identical. Therefore, the optical pathways between thedisplay source 180 and the reflecting surfaces of the converging lenses194L and 194R, as well as the focal lengths of these lenses, should beidentical. In addition, the parity of the number of reflections in thetwo LCCDs 182L and 182R should be equal. As seen in FIG. 21, the numberof reflections is four and two in the LCCDs 182L and 182R respectively,and the required identical parity is achieved.

This optical arrangement could be assembled inside a spectacles frame,to form an optical device, wherein the same image is projected for botheyes 208L and 208R by utilizing only one display light source 180located between the two glasses. Usually, the nose-bridge of aconventional spectacles frame is located a few millimeters above theeyes.

FIG. 22 illustrates how to properly insert the images into the eyes ofthe user. Here, the center of the display light source 180 is locatedabove the centers of the eyes 208L and 208R. In addition, thecoupling-in prisms 184L and 184R are rotated to a geometry wherein thereflecting mirrors of the prisms 184L and 184R are oriented parallel tothe partially reflecting surfaces 36L and 36R, respectively. As aresult, the projections of the central light-waves 21 OL and 21 OR onthe substrate planes are parallel to the major axes of the substrates212L and 212R, respectively, and the image light-waves are coupled intothe substrate where the major axes of the images are inclined a fewdegrees above the horizon. The partially reflecting surfaces 36L and 36Rreflect the coupled images back to their original directions and thecentral image light-waves are again parallel to the horizon. The opticalmodule, which can be added to any conventional frame, can be verycompact and lightweight, with no disturbance to the user.

In the embodiment illustrated in FIGS. 21 and 22 a single display sourceis utilized. As a result, only a system having identical images for botheyes can be materialized using this embodiment An alternate manner ofutilizing two different display sources 214L and 214R, is illustrate inFIG. 23. Here, the light-waves from the display sources 214L and 214Rare coupled into an intermediate prism 216 through surfaces 218L and218R, respectively. The light-waves are then collimated by the LCCDs220R and 220L and trapped inside the substrates 30R and 30L,respectively in a similar manner to that described above with referenceto FIG. 20. A similar structure is illustrated in FIG. 24 wherein thelight-waves coupled into the prism 216, are reflected off the mirror222. Here, the light-waves from the display sources 214L and 214R arecoupled into the substrates 30L and 30R, respectively. Unlike theprevious embodiment which is illustrated in FIGS. 21 and 22, here thedisplay sources are located at the same level as the viewer's eyes,where the space 224 between the LCCDs 220L and 220R is preserved for theupper part of the nose.

The embodiments illustrated in FIGS. 23 and 24 are capable of projectingstereoscopic images, as well as have a wider FOV, by utilizing twodifferent display sources. The necessity to use two different displaysources, however, increases the volume, the power consumption, as wellas the fabrication costs of the optical system. An alternative manner,utilizing only a single display source while still preserving theadvantages of the embodiments of FIGS. 23 and 24, is described in FIG.25. Here, only a single display source is located next to surface 218Land a beamsplitter 226 is inserted into the intermediate prism 216.Apparently, the manner in which a conventional display source 216 andbeamsplitter 226 are utilized, is not possible, since the parity of thetwo images is different, namely, the left image is reflected five times,while the right image is reflected four times before being coupled intothe substrates. The images will be the mirror images of each other.Therefore, to enable the projection of a proper imagery into the eyes, adynamic half-wavelength retardation plate 228 is inserted betweendisplay source 214 and surface 218L, and a polarizing beamsplitter 226is inserted into the prism 216. Here, the display source 214,synchronized with the retardation plate. 228, operates at a double ratecompared to the usual. At the first stage of each double-cycle, an imagewhich is designated to the left eye emanates from the display source. Atthe same time, the dynamic retardation plate is switched to the offposition and the s-polarized light-waves are reflected off thepolarizing beamsplitter 226, collimated by the LCCD 220L, and coupledinto the left substrate 30L. At the second stage of each double-cycle animage which is designated to the right eye emanates from the displaysource. At the same time, the dynamic retardation plate 228 is switchedto the on position, the polarization of the light-waves is rotated tothe p-state and now the p-polarized light-waves pass through thepolarizing beamsplitter 226, collimated by the LCCD 220R and coupledinto the right substrate 30R. The images which are projected into theviewer's eyes can now either be identical, stereoscopic, two differentparts of the same image with some overlap between the two eyes, or anydesired combination.

Hereinbefore it was assumed that the polarizing beamsplitters totallyreflect one polarization and totally transmit the other one. Theoperation of the beamsplitter is not perfect and there is cross-talkbetween the two states. As a result, a small fraction of the s-polarizedlight-waves pass through the beamsplitter and a small fraction of thep-polarized light-waves are reflected off the beamsplitter.

As illustrated in FIG. 26, a part 230, the original s-polarizedlight-waves 2 emanated from the display source 4, passes through thebeamsplitter 10 and is coupled-out of the LCCD 28 at a similar directionto that of the collimated light-wave 232, and therefore, they cannot belaterally separated. In addition, since these two light-waves ares-polarized, they cannot be separated utilizing a polarization sensitiveelement. As a result, the undesired light-waves 230 will be projectedinto the viewer's eye and a ghost image interfering with the originalimage, will be created.

A possible way to overcome this problem is illustrated in FIG. 27. Here,instead of utilizing a single polarizing beamsplitter 10, two differentpolarizing beamsplitters, 234 and 236, and a linear polarizer 238 whichis oriented to block s-polarized light, are disposed into the LCCD 28.Now, the s-polarized light-waves 240 which pass through the firstpolarizing beamsplitter 234 are blocked by the polarizer 238. On theother hand, the light-waves which are reflected as intended by the firstpolarizing beamsplitter, are changed to p-polarized light by theretardation plate 14 and the reflecting surface 16. Following a passagethrough the first beamsplitter 234, the polarizer 236 and the secondpolarizing beamsplitter 238, the light-waves are then reflected,collimated and changed back to s-polarized light by the retardationplate 20 and the optical element 22. Following a reflection off thesecond polarizing beamsplitter 236, the light-waves exit the substrateproperly collimated through the upper surface 26. While theconfiguration illustrated in FIG. 27 solves the ghost image problem, thenecessity to embed two different polarizing beamsplitters and a linearpolarizer inside the LCCD complicates the fabrication process of theoptical module.

Heretofore, it has been assumed that the polarizing beamsplitter isoriented at an angle of 45° in relation to the major surfaces of theLCCD, and that the main output direction from the LCCD (namely, thedirection of the principal ray of the central light-wave) is identical(as in FIG. 1) or normal to (as in FIG. 17) the main input direction onthe LCCD. An alternative manner for separating between the real and theghost images by utilizing a different angle is illustrated in FIG. 28.Accordingly, the angle between the lower surface 8 of the LCCD and thebeamsplitter 10 is set to be 45°+α, wherein a positive angular rotationis defined to be counterclockwise. As a result, the principal ray 242from the light-wave 2 is rotated by the beamsplitter 10 to the angle90°+2α and is reflected to the angle 270°−2α by the reflecting surface16. In order to minimize field aberrations, the main axis of thecollimating optical element 22 is oriented collinear with the incomingprincipal ray 244. Hence, the ray is reflected back by the lens to theangle 90°−2α. The second reflection off the beamsplitter 10 yields thefinal direction of the principal ray to be 4α. Hence, the outputdirection of the principal ray 244 is substantially different than theinput direction of the ray. Therefore, the real and the ghost images cannow be angularly separated.

A slightly modified embodiment is illustrated in FIG. 29 where, inaddition to the rotation of the beamsplitter 10, the reflecting surface16 is rotated at an angle of β compared to its original orientation. Asa result, the direction of the output central ray from the LCCD is now4α+2β. Another example, which is related to the embodiment of FIG. 15,is illustrated in FIG. 30. Here, the angle between the incoming surface246 and the first polarizing beamsplitter is 45°+α and the angle betweenthe reflecting surface 250 and the output surface 252 is a. Therefore,the output direction from the LCCD of the principal ray 253 issubstantially. different than the normal to the input direction of thatray 253 from the LCCD. As a result, the angle between the ghost and thereal images is 2α.

In certain situations positioning the beamsplitter at an orientation of45°+α in relation to the major surfaces of the LCCD does not providesufficient results, regarding the ghost image. A possible way ofovercoming this problem is by combining both methods according to FIG.31. Here, instead of utilizing a single polarizing beamsplitter 248 ofFIG. 30, two different polarizing beamsplitters, 254 and 256, areinserted at an orientation of 45°+α in relation to the major surface ofthe LCCD 258. In this case, even a ghost image which is at an angle of2α with respect to the real image is obstructed by the second polarizingbeamsplitter.

There are various types of polarizing beamsplitters. One type contains alayer of film. This type cannot be utilized in an imaging system becausethe surface quality of the film is usually not sufficient for imaging ofs-polarized light that is reflected off of it. On the other hand,p-polarized light that passes through the film preserves the quality ofthe image, and therefore, this beamsplitter can be used for illuminatingthe light display source. In this case, the beamsplitter cannot have adual function of illumination as well as the imaging functions, such asbeamsplitter 248 shown in FIG. 30.

In FIG. 32, the s-polarized light-waves 258, emanating from a lightsource 260, are reflected off the first polarizing beamsplitter 262 andilluminate the front surface of the LCOS light source 264. Thepolarization of the reflected light from the “light” pixels is rotatedto the p-polarization state. The p-polarized light then passes throughthe polarizing beamsplitter 262 and the light-waves pass through a halfwave retarder 266 and are rotated to s-polarized light. The light isthen reflected off of the second beamsplitter 268 and is coupled out ofthe prism 270 through the lower surface 272 of the prism. Thelight-waves then pass through a quarter-wavelength retardation plate274, reflected by a reflecting optical element 276, return to pass againthrough the retardation plate 274, and re-enter the prism 270 throughthe lower surface 272. The now p-polarized light passes through thesecond polarizing beamsplitter 268 and is coupled out of prism 278through the upper surface 280 of the prism. The light-waves then passthrough a second quarter-wavelength retardation plate 282, collimated bya lens 283 at its reflecting surface 284, return to pass again throughthe retardation plate 282, and re-enter prism 278 through surface 280.The now s-polarized light-waves are reflected off the polarizingbeamsplitter 268 and exit the substrate through the upper surface 285 toenter the substrate 286 such that the light from the display source istrapped inside the substrate by total internal reflection.

An additional configuration meeting the requirement of the beamsplitterhaving a single function of illuminating the display source is describedin FIG. 33. The s-polarized light-waves 288, emanating from a lightsource 290, are reflected off the first polarizing beamsplitter 292 andilluminate the front surface of the LCOS light source 294. Thepolarization of the reflected light from the “light” pixels is rotatedto the p-polarization state. The p-polarized light then passes throughthe polarizing beamsplitter 292 and the light-waves pass through a halfwave retarder 295 and are rotated to s-polarized light. The light isthen reflected off the first reflecting surface 296 and follows totalinternal reflection off the lower surface 298 of the prism 300. Thelight-waves are then reflected off the polarizing beamsplitter 302 andare coupled out through the lower surface 298 of the prism. They passthrough a quarter-wavelength retardation plate 304, are collimated by alens 306 at its reflecting surface 308, return to pass again through theretardation plate 304, and re-enter prism 300 through the lower surface298. The now p-polarized light-waves pass through the polarizingbeamsplitter 302 and exit the substrate through the upper surface 310.

In FIGS. 32 and 33, the s-polarized light that is reflected off thebeamsplitter is reflected before the light impinges the display source,and hence, the first beamsplitter is not limited to a certain type, asopposed to the second beamsplitter, which is limited to a type whichpreserves the quality of the image.

An additional advantage illustrated in FIGS. 28 to 31 is that it ispossible to differentiate between the mechanical orientation of the LCCDand the direction of the output wave. There are cases where it isrequired to rotate the mechanical axis of the LCCD while still retainingthe on-axis impinging angle on the substrate.

As illustrated in FIG. 34, the angle between the lower surface 8 and thebeamsplitter 10 is set to −45°−α. As a result, the angle between theinput and the output light-waves of the LCCD is −4α. Therefore, themechanical angle between the major axis of the LCCD and the normal tothe substrate, is −4α while the central output wave from the LCCDvis-a-vis the central input light-wave of the substrate impinges normalto the major surface of the substrate, as required.

FIG. 35 illustrates another example based on the embodiment which isdescribed above with reference to FIG. 19. Here, the angle betweensurface 8 and the beamsplitter 10 is 52.5°. Therefore, while themechanical axis of the LCCD is oriented at an angle of 30° to normal ofthe major surfaces of the substrate, the central incoming wave isoriented at the required off-axis angle of 60°.

There are applications wherein high-brightness image is required andhence an LCOS light source is the preferred image source instead of theback-illuminated LCD which is illustrated in FIGS. 34 and 35.

As shown in FIG. 36, a cubic polarizing beamsplitter 316 is insertedbetween the display source and the LCCD. Here, the s-polarizedlight-waves 314, emanating from a light source 84, reflect off thepolarizing beamsplitter 312 of a prism 316 and illuminate the frontsurface of the LCOS light source 318. The polarization of the reflectedlight from the “light” pixels is rotated to the p-polarization and thelight-waves are then passed through the beamsplitter 312 and enter theprism 320 through the lower surface 8. The light-waves are thencollimated as described above with reference to FIG. 35. As before, ifit is required that the coupled light 314 into the prism 320 will bes-polarized as in the folding beamsplitter 316, and it is possible toadd a half-wavelength retardation plate 322 between prisms 316 and 320,which rotates the light into the desired polarization.

In the optical systems illustrated in FIG. 36, the optical path isfolded around the y axis, that is, in the x-z plane, wherein the x and yaxes are referred to the horizontal and vertical axes of the image.Usually, it is preferred to fold the optical system in the plane whereinthe dimensions of the system are minimal. As explained above withreference to FIG. 13, for most configurations, the horizontal dimensionis larger than the vertical one in a ratio of 4:3 for VGA format and ina ratio of 16:9 for HDTV format. Therefore, it is preferred to fold theoptical pathway in the prism 316, which is adjacent to the displaysource, around the x axis.

A modified version of FIG. 36, wherein the optical pathway is folded intwo different planes is illustrated in FIGS. 37 (top view), 38 (sideview) and 39 (three dimensional view). Here the folding-in prism 316folds the light in the y-z plane by the polarizing beamsplitter, whilein prism 320 it is folded in the x-z plane. In this configuration, thelight 314 coupled into the prism 320, which is p-polarized compared tothe beamsplitter 312, will be s-polarized compared to the foldingbeamsplitter 320, and no rotating half-wavelength retardation plate isrequired between prisms 316 and 320.

So far, in all the embodiments which were illustrated in FIGS. 2 to 39it has been assumed that the LCCD and the substrate are fabricated fromthe same optical material. There are cases where it is required toutilize different optical materials having different optical propertiesfor at least parts of the LCCD and the substrate. For example, it issometimes necessary to fabricate the LCCD from a specific polymer-basedmaterial, having a refractive index n₁ and an Abbe value v₁ because ofhigh-volume fabrication method restrictions, while it is required tofabricate the substrate from a silicate based material, having adifferent refractive index n₂ and an Abbe value v₂, because of someoptical and system consideration. For some systems, the differences inthe optical properties of the two elements should not cause anyparticular problem. As illustrated in FIGS. 6 to 14 the centrallight-wave intersects the interface plane between the LCCD and thesubstrate normal to the plane. As a result, the direction of theincoming light-wave will not be influenced by the optical differencebetween the two elements. Even for the other light-waves in the image,the intersection angle of the interface plane will be fairly small andthe undesired effects will be negligible.

For the optical systems illustrated in FIGS. 19 and 34 to 39, however,the central wave intersects the interface plane between the LCCD and thesubstrate at a highly oblique angle which is larger than the criticalangle for total internal reflection inside the substrate. As a result,undesired effects may appear which may severely degrade the opticalperformance of these systems.

FIG. 40 illustrates an embodiment of an optical system comprising anoptical device 28 having an input surface 8 and an exit surface 26 forcollimating light waves emitted from a light source 4 and coupled into asubstrate 30 through an optical element 325. The light waves enter thesubstrate 30 via an input aperture 321 and exit the substrate via anoutput aperture 323.

As further illustrated in FIG. 40, a ray 324 from the central wave ofthe image source impinges on the interface plane 326 between the LCCDand the substrate at an off-axis angle α_(in). It is assumed that theLCCD and the substrate have been fabricated of different opticalmaterials having refractive indices and Abbe values of n₁, v₁ andn_(2,v2), respectively. It is also assumed that the ray 324 is acontinuum of wavelengths over the entire photopic spectrum, but for thesimplicity of the illustration only three representative wavelengths:λ_(r); λ_(g) and λ_(b) have been considered. The off-axis angle α′_(in)of coupled ray inside the substrate is given by:

$\begin{matrix}{{\sin\;\alpha_{i\; n}^{\prime}} = {\sin\;{\alpha_{in} \cdot \frac{n_{1}}{n_{2}}}}} & (1)\end{matrix}$Because of the different properties of the optical materials of the LCCDand the substrate, the term n₂/n₁ is usually not uniform for the entirephotopic spectrum, that is:

$\begin{matrix}{\frac{n_{1}\left( \lambda_{1} \right)}{n_{2}\left( \lambda_{1} \right)} \neq \frac{n_{1}\left( \lambda_{g} \right)}{n_{2}\left( \lambda_{g} \right)} \neq \frac{n_{1}\left( \lambda_{b} \right)}{n_{2}\left( \lambda_{b} \right)}} & (2)\end{matrix}$Consequentially:sin α′_(in)(λ₂)≠sin α′_(in)(λ_(g))≠sin α′_(in)(λ_(b))  (3)That is, the single incoming ray 324 is divided into three differentrays 328, 330 and 332 inside the substrate 30 for the three differentwavelengths λ_(r), λ_(g) and λ_(b), respectively. As a result, theoutput light-waves 333 which are coupled out of the substrate 30 aresmeared as a function of the wavelength, and the image quality willsuffer from a severe chromatic aberration.

Another undesired effect is illustrated in FIG. 41. The central point ofthe image source and the two marginal points, which are at the samedistance from the central point, are represented by the three rays, 334,336 and 338, respectively. After the rays. are collimated by the LCCD,the two marginal rays are equally angularly deviated from the centralray, i.e., the incoming angles of the three rays 334, 336 and 338 on theinterface plane 326 are α′_(out), α′_(out)−δ and α′_(out)+δ,respectively. The angular deviation between the incoming ray and thecoupled ray inside the substrate is given by:

$\begin{matrix}{{\Delta\alpha}_{in} = {{\alpha_{in}^{\prime} - \alpha_{in}} \approx \frac{{\sin\;\alpha_{in}^{\prime}} - {\sin\;\alpha_{in}}}{\cos\;\alpha_{in}}}} & (4)\end{matrix}$Inserting Eq. (1) into Eq. (4) yields:

$\begin{matrix}{{{\Delta\alpha}_{in} \approx \frac{{\frac{n_{1}}{n_{2}}\sin\;\alpha_{in}^{\prime}} - {\sin\;\alpha_{in}}}{\cos\;\alpha_{in}}} = {\left( {\frac{n_{1}}{n_{2}} - 1} \right)\tan\;\alpha_{in}}} & (5)\end{matrix}$Similarly, for the two marginal rays, the angular deviations between theincoming rays and the coupled rays inside the substrate are given by:

$\begin{matrix}{{{\Delta\left( {\alpha_{in} - \delta} \right)} \approx {\left( {\frac{n_{1}}{n_{2}} - 1} \right){\tan\left( {\alpha_{in} - \delta} \right)}}}{\Delta\left( {\alpha_{in} + \delta} \right)} \approx {\left( {\frac{n_{1}}{n_{2}} - 1} \right){\tan\left( {a_{in} + \delta} \right)}}} & (6)\end{matrix}$That is, the incoming rays refract differently into the substrate 30 andas a result the angular deviations between the marginal rays 336 and 338and the central ray 334 inside the substrate, are no longer equal.Consequently, the coupled-out image 339 will appear distorted to theviewer's eye.

A possible method of overcoming these problems, at least partially, isillustrated in FIG. 42. Here, the intermediate optical element 325,e.g., a prism 340 between the LCCD and the substrate is fabricated of anoptical material having a refractive index and an Abbe value of n₃ andv₃, respectively. The optical element 325, which is practically a partof the LCCD 28, can be fabricated of an optical material, which issimilar to that of the substrate 30. The interface plane 342 between theoptical element 325 and the LCCD is slanted at an angle β_(int) inrespect to the interface plane 326 between the substrate and the prism.Now, the incoming rays intersect two interface planes before beingcoupled into the substrate. By proper control of the parameters n₃, v₃and β_(int) it is now possible to overcome the problems mentioned above.Naturally, not all the required parameters can be physically ortechnically achieved, for example, by fabricating the intermediate prismfrom the same optical material as the substrate and by choosingβ_(int)=α_(in). Since the central wave intersects the interface plane342 normal to the plane and since the other interface plane is locatedbetween two identical materials, it is possible to eliminate thechromatic aberration as well as the dispersion. Unfortunately, The angleα_(in) is a relatively large angle and setting β_(int)=α_(in) willsignificantly increase the volume of the LCCD, reduce the achievablefield-of-view and considerably degrade the overall optical performanceof the system. In addition, only limited discrete values of n₃ and v₃are physically achievable.

Nevertheless, for most cases, by proper optical design, as illustratedin FIGS. 42 and 43, achievable values of the parameters n₃, v₃ andβ_(int) can be found that will significantly reduce the chromaticaberrations as well as the distortion of the optical system. Asillustrated in FIG. 42, the chromatic aberrations which are created bythe interface planes 326 and 342 are mutually compensated, wherein inFIG. 43 the total distortion which is created by these interface planesis significantly smaller than the distortion which is illustrated inFIG. 41, wherein the distortion is created by a single interface plane326.

FIG. 44 is a three-dimensional illustration of a system where thedisplay source is an LCOS light source which is illuminated by the samemethod as described above with reference to FIGS. 37 to 39. Since theinterface between the LCCD and the substrate in FIG. 44 is identical tothat of an LCD-based system, the design procedure should be also similarhere.

Another situation where a combination of an LCCD and a substrate canyield an optical system which suffers chromatic aberration, as well asdistortion, is illustrated in FIG. 45. The input light-waves are coupledinto the LCCD normal to the input surface 344, the central light-wave isreflected off of the partially reflective surfaces 36 of the substrateat an output direction which is significantly deviated at an angleα′_(out) in respect to the normal of the substrate's plane. Assumingthat the refractive index and the Abbe value of the substrate are n₂ andv₂ and that the light-waves are coupled out from the substrate into theair, the coupling-out angle of the output waves from the substrate are:sin α_(out)=sin α′_(out) ·n ₂  (8)An example for an optical system where the output light-waves aresignificantly oblique to the substrate plane is in a case where it isrequired that the projected image will not block the central angulararea of the viewer. In that case, it is preferred to project the imagefrom the peripheral part of the substrate. Since the optical material ofthe substrate has a finite Abbe value v₂ the output direction depends onthe wavelength of the optical ray, that is:sin α_(out)(λ)=sin α′_(out) ·n ₂(λ)  (9)The term n₂ is usually not uniform for the entire photopic spectrum,that is:n ₂(λ_(r))≠n ₂(λ_(g))≠n ₂(λ_(b))  (10)and consequentially:sin α_(out)(λ_(r))≠sin α_(out)(λ_(g))≠sin α_(out)(λ_(b))  (11)That is, the single incoming ray 346 is divided outside the substrateinto three different rays 348, 350 and 352 outside the substrate for thethree different wavelengths λ_(r), λ_(g) and λ_(b), respectively. As aresult, the output light-waves which are coupled out of the substrateare smeared as a function of the wavelength, and the image quality willsuffer from a severe chromatic aberration.

As illustrated in FIG. 46, another related problem is the distortion.The central point of the image source and the two marginal points whichare at an equal distance from the central point are represented by thethree rays, 354, 356 and 358, respectively. After the rays arecollimated by the LCCD, the two marginal rays are equally angularlydeviated from the central ray. That is, the off-axis angles of the threerays 354, 356 and 358 after the reflection from the partially reflectionsurfaces 36 are α′_(out), α′_(out)−δ and α′_(out)+δ, respectively.Similarly to the derivation of Eq. (7), it can be shown that the angulardeviations between the reflected rays from the partially reflectivesurfaces and the coupled rays outside of the substrate are given by:Δ(α′_(out))≈(n ₂−1)tan(α′_(out))Δ(α′_(out)−δ)≈(n ₂−1)tan(α′_(out)−δ)Δ(α′_(out)+δ)≈(n ₂−1)tan(α′_(out)+δ)  (12)Therefore the off-axis angles of the coupled-out central and the twomarginal rays from the substrate are:sin(α′_(out))+Δ(α′_(out))≈sin(α′_(out))+(n ₂−1)tan(α′_(out))sin(α′_(out)−δ)+Δ(n _(out) ¹−δ)≈sin(α′_(out))+(n ₂−1)tan(α′_(out)−δ)sin(α′_(out)+δ)+Δ(α_(out) ¹+δ)≈sin(α′_(out))+(n₂−1)tan(α′_(out)+δ)  (13)That is, the coupled-out rays refract differently from the substrate andas a result the angular deviations between the marginal rays 356 and 358and the central ray 354 outside the substrate are no longer equal.Consequently, the coupled-out image will appear distorted to theviewer's eye. The chromatic aberrations and the distortion are caused bythe interface between the substrate and the air, namely, the lowersurface 32 of the substrate. These problems, however, are valid for allthe possible configurations of a peripheral image, even when both thesubstrate and the LCCD are fabricated from the same optical material. Inaddition, these problems are also valid not only for systems where thetrapped light-waves intersect the input aperture of the substrate at anoblique angle, as illustrated in FIGS. 40 to 47, but also for systems,e.g., as illustrated in FIGS. 6 to 14, wherein the trapped light-wavesintersect the input aperture of the substrate at an angle smaller thanthe critical angle, for total internal reflection inside the substrateand even for systems where the light-waves intersect the input apertureof the substrate substantially normal to the major plane.

To solve these problems, the same configuration illustrated withreference to FIGS. 42 and 43, is used, namely, an intermediate prism 340which is fabricated of an optical material having a refractive index andAbbe value of n₃ and v₃, respectively, is inserted between the LCCD andthe substrate, wherein the interface plane 342 between the prism 340 andthe LCCD is slanted at an angle β_(int) with respect to the interfaceplane 326 which is located between the substrate and the prism. It isassumed that in the most general case where the LCCD and the substrateare fabricated of the optical materials, where the refractive indicesand Abbe values are n₁; v₁ and n₂; v₂, respectively.

As illustrated in FIG. 47, there are now three different interfaceplanes which refract the input light-waves inside the optical systemdifferently: the interface plane 342 between the LCCD and theintermediate prism. the interface plane 326 between the intermediateprism and the substrate and the lower surface 32 of the substrate, whichcan be regarded as the interface between the substrate and air.Eventually, by proper control of the parameters n₃, v₃ and β_(int), itis possible to decrease these two undesired effects. The figure showsthe combination of the chromatic effects of all three interface planes.The central white ray 346 is refracted by the first interface plane 342into three different rays 348, 350 and 352 which have three differentwavelengths λ_(r), λ_(g) and λ_(b), respectively. Each ray is, however,refracted differently by the two other interface planes, 326 and 32, insuch a way that all these three rays are coupled-out of the substrate atthe same off-axis angle. Hence, the chromatic aberration of the imageray 346 is essentially eliminated.

FIG. 48 illustrates the combination of the distortion effects of allthree interface planes. The central point of the image source and thetwo marginal points which are represented by the three rays 354, 356 and358, respectively, are equally angularly deviated after the collimationby the LCCD. Each of these rays is differently deviated by the threeinterface planes, but the combination of all the three differentdeviations yields a total deviation which is fairly uniform for thedifferent image light-waves. Hence, the total distortion of the opticalsystem is significantly improved as compared to that of FIG. 46, whereinonly one interface plane contributes to the distortion.

For some optical systems relatively narrow FOVs are required for a largedisplay source in order to achieve high resolution. This requires alarge focal length which can result in a large optical system. Oneapproach is to utilize two lenses in the LCCD which achieves a shorteroptical path between the display source and the output surface of theLCCD for the same focal length.

As illustrated in FIG. 49, the s-polarized light-waves 360, emanatingfrom a light display source 362, are reflected off the first polarizingbeamsplitter 364 and are coupled-out of the prism 366 through surface368 of the prism. The light-waves then pass through a quarter-wavelengthretardation plate 370, reflected by a first reflecting optical element372, return to pass again through the retardation plate 370, and reenterthe prism 366 through surface 368. The now p-polarized light-waves passthrough the polarizing beamsplitter 364 and are coupled out of prism 366through surface 374 of the prism. The light-waves then pass through asecond quarter-wavelength retardation plate 376, diverged by a concavelens 378 at its reflecting surface 380, return to pass again through theretardation plate 376, and re-enter prism 366 through surface 374. Thenow s-polarized light-waves are reflected off the polarizingbeamsplitter 364 and exit prism 366 through the upper surface 382 toenter the second prism 384 through its lower surface 386. Thelight-waves are then reflected off the second polarizing beamsplitter388 and are coupled out of the prism 384 through surface 390. Thelight-waves then pass through a third quarter-wavelength retardationplate 392, reflected by a second reflecting optical element 394, returnto pass again through the retardation plate 398, and re-enter prism 384through surface 390. The now p-polarized light-waves pass through thesecond polarizing beamsplitter 388 and are coupled-out of prism 384through surface 396. The light-waves then pass through a fourthquarter-wavelength retardation plate 398, collimated by a convex lens400 at its reflecting surface 402, return to pass again through theretardation plate 398, and re-enter prism 384 through surface 396. Thenow s-polarized light-waves are reflected off the second polarizingbeamsplitter 388 and exit the substrate through the upper surface 404 toenter the substrate 30.

In all the optical systems which were described above in relation toFIGS. 1 to 49, the LCCD operates as a collimator, namely, a real imagefrom a display light source is focused to infinity. In addition, themain purpose for materializing the LCCD is to create collimatedlight-waves as the input for a substrate-based optical system. Clearly,an LCCD could be utilized for different optical operations and manyother applications, i.e., the LCCD can focus an image to a differentdistance than infinity and can be inserted into other systems where itis desired to achieve a satisfactory performance and retain a compactand light-weight system.

FIG. 50 illustrates an optical system where the LCCD serves as afocusing lens for a camera. Here the p-polarized component of the inputwave 406 from an external scene passes through the upper surface 407 ofthe LCCD 408 and through the polarizing beamsplitter 410. It is thenreflected, converged and changed to s-polarized light by the retardationplate 412 and a focusing lens 414 having a reflective back surface 416.Following a reflection off the polarizing beamsplitter 410, it isreflected and changed to p-polarized light by the retardation plate 417and a reflecting surface 418. Following a passage through thebeamsplitter 410 the converging light-waves exit the LCCD through theleft surface 420 and are then focused onto a detector plane 422. A focusmechanism might be added to this device by enabling a lateraltranslation of the camera or of the focusing lens along the z-axis inrelation to the lower plane 424.

Another potential embodiment is illustrated in FIG. 51. Here, aconverging LCCD 426 and a collimating LCCD 428 are combined together toform a unity magnification telescope. An optical filter 430 can beinserted into the Fourier plane, in order to perform any required imageprocessing. In addition, an energy-sensitive filter can be inserted toblock high-intensity signals. The main drawback of the systemillustrated in FIG. 51 is that the image is vertically inverted, i.e.,the transferred image appears upside-down to the viewer's eye.

FIG. 52 illustrates a modified system wherein an inverting prism 432 isadded at the input surface 434 of the unity magnification telescope. Asa result, the image will appear at the proper orientation to theviewer's eye. Different LCCDs can be combined to form magnifying orde-magnifying systems. In addition, the telescope can be combined with asubstrate, in order to project a processed or a magnified image from theexternal view to a viewer's eye.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The invention claimed is:
 1. An optical system, comprising: an opticaldevice (28) for collimating light waves including an arrangement ofoptical elements having a light-waves input surface (8) and alight-waves exit surface (26); a light-waves transmitting substrate (30)having at least two external major surfaces (32, 34) parallel to eachother, edges, an input aperture (321) and an output aperture (323); anoptical element (325) interposed between the exit surface (26) of thecollimating optical device (28) and the input aperture (321) of thesubstrate (30), for coupling the collimated light waves from the opticaldevice (28) into the substrate (30) so that the light waves are trappedinside the substrate by internal reflection; at least one partiallyreflecting surface (36) located in the substrate (30) for coupling thelight waves out of the substrate, the partially reflecting surface (36)being non-parallel to the major surfaces (32, 34) of the substrate (30);the input aperture (321) of the substrate (30) and the optical deviceoptical element (325) defining a first interface plane (326), and thecollimating optical device (28) adjacent to the first interface plane(326) is made of a first optical material, and the substrate (30)adjacent to the first interface plane (326) is made of a second,different optical material, wherein each of the light waves coupled bythe optical element (325) into the substrate (30) is incident on thefirst interface plane (326) at a first off-axis angle α and is refractedinto the substrate at an off-axis angle α′ different from the firstoff-axis angle.
 2. The optical system according to claim 1, wherein theinput aperture of the substrate is optically cemented to the exitsurface of the collimating optical device at the first interface plane.3. The optical system according to claim 1, wherein the differentoptical materials have different optical properties.
 4. The opticalsystem according to claim 1, wherein the different optical materialshave different refractive indices.
 5. The optical system according toclaim 1, wherein the different optical materials have different Abbevalues.
 6. The optical system according to claim 1, further comprising adisplay light source (4) emitting light waves in a photopic spectrumcollimated by the optical device (28) and coupled by the optical element(325) into the substrate so that the collimated light waves are trappedinside the substrate (30) by said internal reflection.
 7. The opticalsystem according to claim 6, wherein the light waves trapped in theoptical device intersect the first interface plane (326) at an obliqueangle.
 8. The optical system according to claim 7, wherein the obliqueangle is greater than a critical angle, for effecting total internalreflection inside the substrate.
 9. The optical system according toclaim 7, wherein the collimated light waves comprise at least twodifferent wavelengths.
 10. The optical system according to claim 9,wherein each of the light waves of the at least two differentwavelengths is refracted into two substantial different directions whilepassing through the first interface plane.
 11. The optical systemaccording to claim 9, wherein the collimating optical device (28) andthe optical element (325) are attached to each other at a secondinterface plane (342).
 12. The optical system according to claim 11,wherein a first external surface of a prism is located adjacent to theinput surface aperture of the substrate and the second interface plane(342) is slanted at an angle β into with respect to the first interfaceplane (326).
 13. The optical system according to claim 11, whereincollimated light waves passing through the first and second interfaceplanes (326, 342) suffer chromatic aberrations.
 14. The optical systemaccording to claim 9, wherein for the two different wavelengths each ofthe light waves is refracted into two substantial different directionswhile passing through a second interface plane (342).
 15. The opticalsystem according to claim 6, wherein at least a first (334), a second(336) and a third (338) light rays are emanating from the display source4, the first (334), second (336) and third (338) light rays collimatedby the collimating device (28) into three different collimated lightrays that propagate into three different directions with an angulardeviation between each two of the collimated rays.
 16. The opticalsystem according to claim 15, wherein the angular deviation between thefirst (334) and the second (336) light rays is substantially differentthan the angular deviation between the first (334) and the third (338)light rays while impinging on the first interface plane (326).
 17. Theoptical system according to claim 16, wherein the angular deviationbetween the first (334) and the second (336) light rays is equal to theangular deviation between the first (334) and the third (338) light raysafter passing through the first interface plane (326).
 18. The opticalsystem according to claim 17, wherein first (334), second (336) andthird (338) light rays passing through the first interface surface (326)suffer distortion, wherein the light rays passing through the secondinterface (342) surface suffer distortion, and wherein the distortion ata second interface plane (342) is compensated by the distortion at thefirst interface plane (326).
 19. The optical system according to claim16, wherein the angular deviation between the first (334) and the second(336) light rays is equal to the angular deviation between the first(334) and the third (338) light rays while impinging on the outputaperture (323).
 20. The optical system according to claim 19, whereinthe angular deviation between the first (334) and the second (336) lightrays is substantially different than the angular deviation between thefirst (334) and the third (338) light rays after passing through theoutput aperture (323).
 21. The optical system according to claim 19,wherein the light rays passing through the first interface plane (326)suffer distortion, the light rays passing through the output aperture(323) suffer distortion and the distortion at the first interface plane(326) is compensated by the distortion at the output aperture (323). 22.The optical system according to claim 21, wherein the light rays passingthrough a second interface plane (342) suffer distortion and thedistortion at the first interface plane (326), the distortion at thesecond interface plane (342) and the distortion at the output apertureare (323) mutually compensated.
 23. The optical system according toclaim 15, wherein the angular deviation between the first (334) and thesecond (336) light rays is equal to the angular deviation between thefirst (334) and the third (338) light rays while impinging on a secondinterface plane (342).
 24. The optical system according to claim 23,wherein the angular deviation between the first (334) and the second(336) light rays is substantially different than the angular deviationbetween the first (334) and the third (338) light rays after passingthrough the second interface plane (342).
 25. The optical systemaccording to claim 15, wherein the angular deviation between theinterface plane first and second interface plane (326, 342) and opticalproperties of the optical element (325) and the collimating opticaldevice (28) are selected to minimize distortion of the optical system.26. The optical device according to claim 15, wherein the first, secondand third light rays (334, 336, 338) are coupled into the collimatingoptical device (28) substantially normal to the input surface (8) of thedevice, and a central light ray (334) is coupled out from thelight-transmitting substrate (30) at an output direction which deviateswith respect to normal of major surfaces (32, 34) of the substrate. 27.The optical system according to claim 26, wherein for the two differentwavelengths each of the light rays are refracted into two substantialdifferent directions while passing through the output aperture (323).28. The optical system according to claim 26, wherein the light rayspassing through the first interface plane (326) suffer chromaticaberrations, the light rays passing through output aperture (323) sufferchromatic aberrations and the chromatic aberrations at the firstinterface plane (326) are compensated by the chromatic aberrations atthe output aperture (323).
 29. The optical system according to claim 28,wherein the light rays passing through a second interface plane (342)suffer chromatic aberrations, and the chromatic aberrations at the firstinterface plane (326), the chromatic aberrations at the second interfaceplane (342) and the chromatic aberrations at the output aperture (323)are mutually compensated.
 30. The optical system according to claim 1,wherein the optical element (325) is made of the second opticalmaterial.